Wafer stacking is recognized as a promising technology for making compact, low cost optoelectronic and fiberoptic devices. Microlenses made using photolithography are fundamental components of this technology. For a concise introduction of the technology, See, for example, a review article by K. Iga and S. Misawa entitled “distributed-index planar microlens and stacked planar optics: a review of progress”, published in APPLIED OPTICS, Vol. 25, No. 19, pp. 3388-3396 (1986). When designing devices using this technology, there is a need for two fibers to share the same microlens, as in dual fiber collimator applications for beam splitters and integrated fiber filters (FIG. 20 in aforementioned article).
FIG. 1 shows a dual fiber collimator according to copending U.S. patent application Ser. No. 09/327,826, in which a fused silica refractive microlens wafer 140 is precisely aligned and bonded to a silicon wafer 130 which has two photolithographically defined and etched through holes called fiber sockets 120 which are used to precisely align the optical fibers 100 to the microlens 150. The optical fibers 100 are inserted into the bottom of the fiber sockets 120. Index-matching epoxy 110 is used to fill any gap between the fused silica wafer 140 and the fiber 100. The microlens 150 is designed so that its focal plane is at the back surface of the microlens wafer 140. Optical signals 170 (ray traces) emitting from the fiber cores 160 are collimated by the microlens 150. The microlens diameter should be large enough to enclose the light emission from both fibers within the low aberration portion of the lens near its center. For the case of two 125 micron diameter fibers adjacent to each other with a microlens wafer 140 thickness of 800 microns, the microlens 150 diameter should be about 335 microns or more.
A basic requirement of the wafer stacking technology is that the final thickness of the wafer stack be as small as possible, since the wafer stack is to be diced into small chips. A thick wafer stack makes dicing into very small pieces without adversely affecting device performance or mechanical integrity difficult. Since the wafer stack thickness will determine the minimum chip size, a thinner microlens wafer 140 in the wafer stack will result in a potentially higher number of chips per wafer, and correspondingly lower cost per chip. A typical microlens wafer thickness may be 800 microns.
The thickness of the microlens wafer 140 also determines the optical beam diameter. The beam diameter is linearly proportional to thickness of the microlens wafer 140 and inversely proportional to the refractive index of the microlens wafer 140. A large diameter of the collimated light beam is critical in wafer stacking technology, since the diameter of the collimated beam determines the collimated distance of the beam, according to light diffraction. A large beam diameters allows long working distances and low diffraction loss. To obtain a large beam diameter, a material with low refractive index such as fused silica or glass is preferred since the light beam emerging from the optical fiber 100 expands in the shortest distance in low index material.
The large microlens diameter and the small thickness of the microlens wafer dictates a high numerical aperture microlens made using glass or fused silica as the substrate for the microlens wafer 140.
There are currently three kinds of microlenses made using photolithographic techniques: refractive, diffractive, and planar diffused microlenses.
Refractive microlenses are made by forming convex surface profiles which provides the light transforming function. The convex profile may be formed by dry etching with either a reflow photoresist mask or a gray scale photoresist mask, which is described in an article entitled “micro-optic fabrication using one-level gray-tone lithography”, by K. Reimer et. al., in SPIE Vol. 3008, pp. 279-288, 1997. The gray scale mask technique is a general photolithographic technique for fabricating arbitrarily shaped surfaces on a wafer. Photoresist is exposed with a spatially varying intensity which controls the photoresist thickness after development. For example, a slanted surface, which is used later in this invention, may be made using a monotonically increasing spatial intensity profile. Refractive microlenses have high performance and low spherical aberration, especially near the center of the microlens. The uniformity of refractive microlenses on a wafer is also excellent.
Planar diffused microlenses are made by diffusing ions into a suitable glass material using an annular metal mask to pattern the diffusion. The ion-exchanged glass has higher refractive index. Planar diffused microlenses can be made easily into large microlenses. However, the optical performance of the planar diffused microlens remains inferior to some other forms of microlens even after twenty years of development. The focal lengths of planar diffused microlenses are difficult to control and not sufficiently uniform across a wafer for fiber optic applications.
Diffractive microlenses are made using binary photomasks. A key advantage of the diffractive microlens is its large diameter. However, diffractive microlenses have higher optical losses than refractive microlenses.
The refractive microlens has the best optical performance among the three microlens types. However, it has been very difficult to make refractive microlens arrays with high numerical aperture and large diameter in fused silica due to the large microlens sag and the slow etch rate.
For the refractive microlens 150, the sag, or depth, s is related to the microlens diameter D through:   s  =      R    -                            R          2                -                              D            2                    4                    where:   R  =                    (                  n          -          1                )            ⁢      L        n  is the microlens radius of curvature,                n is the index of refraction of the microlens material,        L is the thickness of the microlens wafer,        D is the microlens diameter.        
At a wavelength of 1.5 micron, fused silica has an index of refraction of 1.46. A fused silica microlens of 335 micron diameter focusing to the back surface of an 800 micron thick fused silica wafer has a calculated sag of 63.7 microns. Given a typical fused silica etch rate of 300 angstrom per minute, 35 hours of etching would be required to etch this microlens.
Accordingly, there is a continuing need in the industry for a faster, more efficient and robust method for making large, high numerical aperture refractive microlenses.